Acoustic resonator performance enhancement using selective metal etch

ABSTRACT

An acoustic resonator comprises: a substrate; a first electrode adjacent the substrate; a layer of piezoelectric material adjacent the first electrode; a second electrode adjacent the layer of piezoelectric material; and a passivation layer adjacent the layer of piezoelectric material. The passivation layer includes a recessed feature. Fill material is provided in the recessed feature of the passivation layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is related to U.S. patent application Ser. No.10/867,540, filed on Jun. 14, 2004, entitled “ACOUSTIC RESONATORPERFORMANCE ENHANCEMENT USING RECESSED REGION,” now U.S. Pat. No.7,161,488 is commonly assigned to the same assignee as the presentinvention. This patent application is a divisional patent application ofand claims priority from U.S. patent application Ser. No. 11/021,085(now U.S. Pat. No. 7,791,434), filed on Dec. 22, 2004. This patientapplication is assigned to the same assignee as U.S. Pat. No. 7,791,434.

BACKGROUND

The need to reduce the cost and size of electronic equipment has createda need for smaller single filtering elements. Thin-Film Bulk AcousticResonators (FBARs) and Stacked Thin-Film Bulk Wave Acoustic Resonators(SBARs) represent one class of filter elements with potential formeeting these needs. These filters can collectively be referred to asFBARs. An FBAR is an acoustic resonator that uses bulk longitudinalacoustic waves in thin-film piezoelectric (PZ) material. Typically, anFBAR includes a layer of PZ material sandwiched between two metalelectrodes. The combination PZ material and electrodes are suspended inair by supporting the combination around its perimeter or are placedover an acoustic mirror.

When an electrical field is created between the two electrodes, the PZmaterial converts some of the electrical energy into mechanical energyin the form of acoustic waves. The acoustic waves propagate in the samedirection as the electric field and reflect off the electrode-air orelectrode-acoustic mirror interface at some frequency, including at aresonance frequency. At the resonance frequency, the device can be usedas an electronic resonator. Multiple FBARs can be combined such thateach are elements in RF filters.

Ideally, the resonant energy in the filter elements is entirely“trapped” in the resonator. In practice, however, dispersive modesexist. These modes can result in a decreased quality factor (Q) for thefilter.

For these and other reasons, a need exists for the present invention.

SUMMARY

One aspect of the present invention provides an acoustic resonator thatincludes a substrate, a first electrode, a layer of piezoelectricmaterial, and a second electrode. The substrate has a first surface andthe first electrode is adjacent the first surface of the substrate. Thelayer of piezoelectric material is adjacent the first electrode. Thesecond electrode is adjacent the layer of piezoelectric material, andthe second electrode lies in a first plane and has an edge. The layer ofpiezoelectric material has a recessed feature adjacent the edge of thesecond electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a top plan view of an FBAR.

FIG. 2 illustrates a cross-sectional view of an FBAR.

FIG. 3 illustrates a cross-sectional view of an FBAR according to oneembodiment of the present invention.

FIG. 4 illustrates a top plan view of one embodiment of the FBARillustrated in FIG. 3.

FIG. 5 illustrates a top plan view of an alternative embodiment of theFBAR illustrated in FIG. 3.

FIG. 6 illustrates Q circles for two exemplary FBARs plotted on a Smithchart.

FIG. 7 illustrates a cross-sectional view of an FBAR according to oneembodiment of the present invention.

FIG. 8 is a cross-sectional view of an FBAR according to anotherembodiment of the present invention.

FIG. 9 is a cross-sectional view of an FBAR according to anotherembodiment of the present invention.

FIG. 10 is a cross-sectional view of an FBAR according to anotherembodiment of the present invention.

DETAILED DESCRIPTION

In the following Detailed Description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. In this regard, directional terminology, such as “top,”“bottom,” “front,” “back,” “leading,” “trailing,” etc., is used withreference to the orientation of the Figure(s) being described. Becausecomponents of embodiments of the present invention can be positioned ina number of different orientations, the directional terminology is usedfor purposes of illustration and is in no way limiting. It is to beunderstood that other embodiments may be utilized and structural orlogical changes may be made without departing from the scope of thepresent invention. The following detailed description, therefore, is notto be taken in a limiting sense, and the scope of the present inventionis defined by the appended claims.

FIGS. 1 and 2 illustrate top and cross-sectional views, respectively, ofFBAR 10. FBAR 10 includes substrate 12, depression 14, first electrode16, piezoelectric (PZ) layer 18, second electrode 20, and passivationlayer 22. In FIG. 1, passivation layer 22 has been removed, and firstelectrode 16 and depression 14 are hidden from view. Second electrode 20has a perimeter that is illustrated in FIG. 1 as pentagon-shaped.Typically, contacts are coupled to first electrode 16 and to secondelectrode 20. These contacts facilitate connecting the first and secondelectrodes 16 and 20 to a source of voltage. Two locations along thatperimeter, first edge 20 a and second edge 20 b, are illustrated in theplan view of FIG. 1 and the cross-sectional view of FIG. 2.

First electrode 16, PZ layer 18, second electrode 20, and passivationlayer 22 collectively form FBAR membrane 23. FBAR membrane 23 isadjacent substrate 12 and suspended over depression 14 to provide anelectrode-air interface. In one embodiment, depression 14 is created byetching away a portion of substrate 12. Depression 14 is deep enough sothat sufficient electrode-air interface is created under FBAR membrane23.

In an alternative embodiment, FBAR membrane 23 may be placed adjacent anacoustic mirror (not illustrated in FIGS. 1 and 2) formed withinsubstrate 12. In this way, an electrode-acoustic mirror interface isformed. The resonator thus formed is a Solid Mounted Resonator (SMR).

In one embodiment, substrate 12 is made of silicon (Si) and PZ layer 18is made from aluminum nitride (AlN). Alternatively, other piezoelectricmaterials may be used for PZ layer 18. In one embodiment, first andsecond electrode 16 and 20 may be made of molybdenum (Mo).Alternatively, other materials may be used for the electrodes. In oneembodiment, the contacts may be made of gold (Au). Alternatively, othermaterials may be used for the contacts.

FBAR 10 illustrated in FIGS. 1 and 2 is configured to use bulkcompression or sheer acoustic waves propagating in PZ layer 18. When anelectric field is created between first and second electrodes 16 and 20via an impressed voltage, the piezoelectric material of PZ layer 18converts some of the electrical energy into mechanical energy in theform of acoustic waves. So configured, FBAR 10 exhibits dispersive modesresulting in a quality factor (Q) loss for FBAR 10.

FIG. 3 illustrates a cross-sectional view of FBAR 40 in accordance withone embodiment of the present invention. FBAR 40 includes substrate 42,depression 44, first electrode 46, piezoelectric (PZ) layer 48, secondelectrode 50, and passivation layer 52. Typically, contacts (notillustrated in FIG. 3) are coupled to first and second electrodes 46 andelectrode 50. The contacts facilitate connecting first and secondelectrodes 46 and 50 to a voltage source. First electrode 46, PZ layer48, second electrode 50, and passivation layer 52 collectively form FBARmembrane 53, which may be placed over a depression 44 or over anacoustic mirror as discussed above. FBAR membrane 53 is illustratedadjacent substrate 42 and suspended over depression 44 to provide anelectrode-air interface. As with previous embodiments, anelectrode-acoustic mirror interface is also obtainable using an SMRdesign in accordance with the present invention.

Second electrode 50 and passivation layer 52 have a perimeter that canbe of various configurations. For example, the perimeters of each can bepentagon-shaped, similar to FBAR 10 above. They could also be any ofvarious polygonal shapes, circular, or various irregular shapes. Thecross-sectional view illustrated in FIG. 3 illustrates two locationsalong the perimeter of second electrode 50, first and second edges 50 aand 50 b. In one embodiment, edges of passivation layer 52 are generallyaligned with those of second electrode 50 in the vertical direction asFBAR 40 is illustrated in FIG. 3.

In FBAR 40 illustrated in FIG. 3, a recessed feature 60 has beenselectively etched into PZ layer 48 adjacent the first and second edges50 a and 50 b of second electrode 50. When first and second edges 50 aand 50 b of second electrode 50 are considered to be vertical (asoriented in the illustration of FIG. 3), recessed feature 60 is“outside” the first and second edges 50 a and 50 b of second electrode50 in the horizontal direction. (For comparison, depression 44 would beconsidered “inside” the first and second edges 50 a and 50 b of secondelectrode 50 in the horizontal direction.

Recessed feature 60 improves the performance of FBAR 40, resulting inimproved insertion loss and improved resonator quality factor Q of FBAR40. The overall quality factor Q of FBAR 40 depends proportionally on aparameter of resistance called R_(p). In FBAR 40, the R_(p) may beimproved by recessed feature 60.

An electric field is created between first and second electrodes 46 and50 via an impressed voltage. The piezoelectric material of PZ layer 18converts some of the electrical energy into mechanical energy in theform of acoustic waves. Some of the acoustic waves in FBAR 40 arelongitudinally-directed acoustic waves of any mode type, while othersare referred to transversely-directed acoustic waves of the compression,or sheer-mode type. FBAR 40 is designed to use bulk compression or sheeracoustic waves propagating in a longitudinal direction in the PZ layer48 as the desired resonator mode. However, FBAR 40, which providesrecessed feature 60, reduces or suppresses energy loss, therebyimproving the Q of the filter. Recessed feature 60 may also helpsuppress noise in the filter.

Recessed feature 60 may have a depth in PZ layer 48 that is on the orderof hundreds to thousands of angstroms, and may have a width on the orderof fractions of a micron to microns or even larger, up to the width ofPZ layer 48 that extends beyond first and second edges 50 a and 50 b ofsecond electrode 50. In one embodiment, PZ layer 48 is selectivelyetched to form recessed feature 60 that is fraction of a micron tomicrons to 10's of microns wide by 100's to 1000's angstroms deep. Alsoin one case, recessed feature 60 is offset from first and second edges50 a and 50 b of second electrode 50 by fractions of a micron tomicrons. In other words, recessed feature 60 is outside first and secondedges 50 a and 50 b of second electrode 50 by fractions of a micron tomicrons.

FIGS. 4 and 5 illustrate plan views of FBAR 40 of FIG. 3 in accordancewith alternative embodiments of the present invention. As illustrated inFIGS. 4 and 5, FBAR 40 includes substrate 42, piezoelectric (PZ) layer48, second electrode 50. In FIGS. 4 and 5, passivation layer 52 has beenremoved, and first electrode 46 and depression 44 are hidden from view.Typically, contacts (not illustrated in the Figures) are coupled tofirst and second electrodes 46 and electrode 50.

In FIGS. 4 and 5, recessed feature 60 is illustrated extending adjacentthe perimeter of second electrode 50. In the Figures, the perimeter ofsecond electrode 50 is generally pentagon-shaped having five relativelystraight edges, but may also be polygonal in shape, circular in shape,or have any other smooth or irregular shape. In FIG. 4, recessed feature60 is illustrated extending adjacent and outside the perimeter of secondelectrode 50 along four of the five edges of the pentagon-shapedelectrode. Since a contact would typically be attached to the fifth edgeof the electrode (labeled 50 a), recessed feature 60 does not extendalong that edge. FIG. 5 illustrates an alternative embodiment of FBAR 40where recessed feature 60 extends adjacent the perimeter of secondelectrode 50 along two of the five edges of the pentagon-shapedelectrode.

As one skilled in the art will understand, any number of alternativerecessed feature 60 may be provided adjacent the edges of secondelectrode 50 consistent with the present invention. Recessed feature 60may be continuously extending along some or all of the edges of secondelectrode 50 as illustrated, the recessed feature 60 may have smallersegments that are not continuous along the edge, and other shapes andconfigurations of recessed feature 60 may be used, especially wheresecond electrode 50 is a shape other than a pentagon.

FIG. 6 illustrates Q circles for two exemplary FBARs plotted on a Smithchart, and illustrates improvement in R_(p) and therefore Q in one ofthe FBARs. As is known in the art, a Smith Chart is a polar plot of acomplex impedance (used in the following to show measures of s11 and s22scattering parameters). These s11 and s22 scattering parametersrepresent a ratio of complex amplitudes of backward and forward waves.The Smith Chart aids in translating the reflection coefficients intoimpedance and it maps part of the impedance placed into a unit circle.

The improved performance of FBAR 40 is demonstrated by the Q circlesillustrated in FIG. 6. FIG. 6 illustrates the S-parameter measurementsof an exemplary etched device, such as FBAR 40 with recessed feature 60having exemplary measurements in one case that are 14.5 μm wide by 3000Å deep and offset from second electrode 50 by 0.5 μm. As illustrated,etched device (dashed line labeled S11) has a much improved R_(p) versusthat of a control device (solid line labeled S22).

Generally, the horizontal axis passing through the unit circlerepresents real impedance, the area above the axis represents inductivereactance and the area below represents capacitive reactance. Theleft-hand portion of the chart at zero reactance represents seriesresonance frequency (fs) and occurs where the Q circle crosses the realaxes on the left side of the Smith Chart. The left-hand portion of thechart also demonstrates the parameter of resistance R_(s). Theright-hand portion of the chart at zero reactance represents parallelresonant frequency (fp) and occurs where the Q circle crosses the realaxes on the right side the Smith Chart. The right-hand portion of thechart also demonstrates the parameter of resistance R_(p). The closerthat a plot of FBAR filter characteristics on a Smith Chart is to theperimeter of the Smith Chart, the higher the Q will be for that FBAR.Also, the more smooth that the curve is, the lower the noise is in theFBAR.

In FIG. 6, the performance of FBAR 40 as a filter is illustrated bydashed line Q circle s11 and the performance of a prior art FBAR withouta recessed region is illustrated by solid line Q circle s22. As evident,FBAR 40 improves the quality of the filter near the resistance R_(p).FBAR 40, illustrated by Q circle s11, more closely approximates a circleand is representative of a lower noise and less lossy device, whichimproves the performance of FBAR 40 when used in a filter.

FIG. 7 illustrates a cross-sectional view of FBAR 40 in accordance withan alternative embodiment of the present invention. FBAR 40 isessentially the same as that illustrated in FIG. 3, and includessubstrate 42, depression 44, first electrode 46, piezoelectric (PZ)layer 48, second electrode 50, and passivation layer 52. First andsecond edges 50 a and 50 b of the perimeter of second electrode 50 arealso illustrated. In addition, however, FBAR 40 illustrated in FIG. 7,has recessed feature 60 formed on a surface of PZ layer 48 that isopposite the surface in which recessed feature 60 was formed in FIG. 3.As FBAR 40 is depicted in FIG. 3, recessed feature 60 is on the “top”surface of PZ layer 48, whereas as FBAR 40 is depicted in FIG. 7,recessed feature 60 is on the “bottom” surface of PZ layer 48. As withrecessed feature 60 depicted in FIG. 3, recessed feature 60 depicted inFIG. 7 is also outside first and second edges 50 a and 50 b of secondelectrode 50. The performance of FBAR 40 as illustrated in FIG. 7 may bethe same as that described above for FBAR 40 as depicted in FIG. 3.Recessed feature 60 on the “bottom” surface of PZ layer 48 may beachieved in a variety of ways known by those skilled in the art,including but not limited to, a release of a sacrificial material.

FIG. 8 illustrates a cross-sectional view of FBAR 40 in accordance withan alternative embodiment of the present invention. FBAR 40 isessentially the same as that illustrated in FIG. 3, and includessubstrate 42, depression 44, first electrode 46, piezoelectric (PZ)layer 48, second electrode 50, and passivation layer 52. In addition,however, FBAR 40 illustrated in FIG. 8, has fill material 61 inserted inrecessed feature 60, which were illustrated in FIG. 3. Fill material 61inserted in recessed feature 60 can further improve the performance ofFBAR 40, resulting in improved insertion loss and improved resonatorquality factor Q of FBAR 40.

In one embodiment, fill material 61 is the same material as that usedfor second electrode 50. In that case, fill material 61 will havedifferent dispersion characteristics than will the remaining material ofPZ layer 48, which in one case is AlN. Adding this material withdiffering dispersion characteristics can improve insertion loss andimprove the resonator quality factor Q of FBAR 40.

For example, fill material 61 can be made of Mo material just as firstand second electrodes 46 and 50, or metal such as Pt, W, Cu, Al, or Ag.In alternative embodiments, fill material 61 could also be made of othermaterials, such as polyimide, BCB (benzocyclobutene), SiO₂, Si₃N₄, orother dielectrics, AlN, ZnO, LiNbO₃, PZT, LiTaO₃,Al₂O₃, or otherpiezoelectric materials.

FIG. 9 illustrates FBAR 70 in accordance with an alternative embodimentof the present invention. FBAR 70 includes substrate 72, depression 74,first electrode 76, piezoelectric (PZ) layer 78, second electrode 80,and passivation layer 82. Typically, contacts (not illustrated in FIG.9) are coupled to first and second electrodes 76 and electrode 80. Thecontacts facilitate connecting first and second electrodes 76 and 80 toa voltage source. First electrode 76, PZ layer 78, second electrode 80,and passivation layer 82 collectively form FBAR membrane 83, which maybe placed over a depression 74 or over an acoustic mirror as discussedabove. FBAR membrane 83 is illustrated adjacent substrate 72 andsuspended over depression 74 to provide an electrode-air interface. Aswith previous embodiments, an electrode-acoustic mirror interface isalso obtainable using an SMR design in accordance with the presentinvention.

First and second edges 80 a and 80 b of second electrode 80 are alignedin the horizontal direction relative to the edges of passivation layer82. Adjacent these edges, recessed feature 90 is formed in passivationlayer 82. Like recessed feature 60 described previously with respect toFBAR 40, recessed feature 90 improve the performance of FBAR 70,resulting in improved noise reduction and improved resonator qualityfactor Q of FBAR 70.

FIG. 10 illustrates a cross-sectional view of FBAR 70 in accordance withan alternative embodiment of the present invention. FBAR 70 isessentially the same as that illustrated in FIG. 9, and includessubstrate 72, depression 74, first electrode 76, piezoelectric (PZ)layer 78, second electrode 80, and passivation layer 82. In addition,however, FBAR 70 illustrated in FIG. 10, has fill material 91 insertedin recessed feature 90, which is illustrated in FIG. 9. Fill material 91inserted in recessed feature 90 can further improve the performance ofFBAR 70, resulting in improved insertion loss and improved resonatorquality factor Q of FBAR 70. In addition, fill material 91 inserted inrecessed feature 90 of passivation layer 80 affect mass-loading of theresonator in local regions of FBAR 70. This in turn alters the resonantbehavior in that local region. With the correct size and amount ofmaterial absent or present, the overall device performance can bealtered.

In one embodiment, fill material 91 is that same material as that usedfor second electrode 80. In that case, fill material 91 will havedifferent dispersion characteristics than will the remaining material ofPZ layer 78, which in one case is AlN. Adding this material withdiffering dispersion characteristics can improve insertion loss andimprove the resonator quality factor Q of FBAR 70. For example, fillmaterial 91 can be made of Mo material just as first and secondelectrodes 76 and 80, or any metal such as Pt, W, Cu, Al, Au, or Ag. Inalternative embodiments, fill material 91 could also be made of othermaterials, such as polyimide, BCB, SiO₂, Si₃N₄, or other dielectrics,AlN, ZnO, LiNbO₃, PZT, LiTaO₃, Al₂O₃, or other piezoelectric materials.

The process for fabricating FBARs 40 and 70 may be accomplished in avariety of ways consistent with the present inventions. In oneembodiment, FBAR 40 or 70 is built with a standard flow up to a regularfield etch step, the FBAR is masked for the etch feature, then etched,then the resist mask is stripped away and then the regular field etch isfinalized. In this case, field etch refers to etch of the mainpiezoelectric layer.

In another embodiment, FBAR 40 or 70 is built with a standard flow up toa regular field etch step, the FBAR is masked for the etch feature, thenetched, then the resist mask is stripped away. Then a pattern lift-offmask is put down, the fill material is deposited in the etch feature,the lift-off mask is stripped and then the regular field etch isfinalized.

In another embodiment, FBAR 40 or 70 is built with a standard flow up toa regular field etch step, the FBAR is masked for the etch feature, thenetched, then the resist mask is stripped away. Then fill material isdeposited in the etch feature, a etch mask for the fill material is putdown, the fill material is etched, the etch mask is stripped, and thenthe regular field etch is finalized.

The recess in FBAR 70 can be made with an etch step, but can also begenerated by a lift-off process. Furthermore, the recess in FBAR 70 canin some instances be made before the field etch.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

1. An acoustic resonator comprising: a substrate; a first electrodeadjacent the substrate; a layer of piezoelectric material adjacent thefirst electrode; a second electrode adjacent the layer of piezoelectricmaterial; a passivation layer adjacent the second electrode, wherein thepassivation layer includes a recessed feature; and fill material in therecessed feature of the passivation layer, wherein the fill material isnot electrically connected to the acoustic resonator.
 2. The acousticresonator of claim 1 wherein the second electrode comprises a materialthat is the same as the fill material in the recessed feature.
 3. Theacoustic resonator of claim 1 wherein the fill material is a materialselected from the group comprising dielectrics, metals, piezoelectrics,Mo, Pt, Al, Cu, W, Au, Ag, polyimide, BCB (benzocyclobutene), SiO₂,Si₃N₄, AlN, ZnO, LiNbO₃, PZT, LiTaO₃, or Al₂O₃.
 4. The acousticresonator of claim 1 wherein the recessed feature has a depth in thepassivation layer that is on the order of ten to thousands of angstromsand a width on the order of fractions of a micron to microns.
 5. Anacoustic resonator comprising: a substrate; a first electrode adjacentthe substrate; a layer of piezoelectric material adjacent the firstelectrode; a second electrode adjacent the layer of piezoelectricmaterial; a passivation layer adjacent the second electrode, wherein thepassivation layer includes a recessed feature; and fill material in therecessed feature of the passivation layer, wherein the recessed featureextends around a substantial portion of the perimeter of the passivationlayer.
 6. The acoustic resonator of claim 5, wherein the secondelectrode comprises a material that is the same as the fill material inthe recessed feature.
 7. The acoustic resonator of claim 5, wherein thefill material is a material selected from the group comprisingdielectrics, metals, piezoelectrics, Mo, Pt, Al, Cu, W, Au, Ag,polyimide, BCB (benzocyclobutene), SiO₂, Si₃N₄, AlN, ZnO, LiNbO₃, PZT,LiTaO₃, or Al₂O₃.
 8. The acoustic resonator of claim 5, wherein therecessed feature has a depth in the passivation layer that is on theorder of ten to thousands of angstroms and a width on the order offractions of a micron to microns.